In the field of substrates for optics, electronics or opto-electronics, two main types of methods are well known for forming a thin layer on a supporting substrate. According to a first type of method, a thin layer taken from a donor substrate is transferred onto a receiving supporting substrate to obtain substrates including a thin useful layer. Useful layer is the layer of the substrate on which electronic components such as, for example, light-emitting diodes or other components may be made.
According to second type of method, the thin layer is deposited on a receiving supporting substrate by a deposition technique. This deposition technique may notably consist of epitaxy or chemical vapor deposition. Regardless of the type of method used for forming a useful layer on a receiving supporting substrate, in some instances it is necessary to remove at least one portion of the receiving support to obtain a final substrate including at least the useful layer. Such removal of the receiving support results in loss of materials, thereby putting a strain on the manufacturing costs of such substrates.
In order to find a remedy to this drawback, a method for making substrates has been devised which includes a useful thin layer method in which the receiving supporting substrate is removed in order to be recycled. Such a method is described in an alternative embodiment of U.S. Pat. No. 6,794,276, which describes a method for making substrates. This method includes a step for transferring a seed layer on a receiving support by molecular adhesion at a bonding interface, a step for epitaxy of a useful layer on the seed layer and a step for applying stresses in order to lead to removal of the assembly (i.e., removal of the seed layer and of the useful layer from the receiving support at the bonding interface). Seed layer is the material layer which allows development of the epitaxied useful layer.
In U.S. Pat. No. 6,794,276, certain specifications are required for allowing the seed layer to adapt to thermal expansions of the receiving support and the useful layer during heat treatments to which the substrate is subject. For this purpose, it is recommended that the seed layer has sufficiently small thickness, of the order of 0.5 microns, and preferably less than 1,000 Å. U.S. Pat. No. 6,794,276 also mentions the fact that the receiving support consists of a material for which the thermal expansion coefficient is 0.7 to 3 times larger than that of the useful layer. It is specified that the thermal expansion coefficient is the proportionality coefficient of the change in the length of a solid as a function of the initial length of the solid and of its change in temperature according to the following formula:ΔL=αL0ΔT where α=thermal expansion coefficient
In an alternative embodiment, the method taught by U.S. Pat. No. 6,794,276 allows the receiving supporting substrate to be reused after its removal.
It is desirable to improve the method taught by U.S. Pat. No. 6,794,276. In particular, improvements are needed for reducing the risk of breaking the substrate, deteriorating, cracking the seed layer or the occurrence of a residual deflection of the final substrate making it unusable during the various heat treatments applied to the substrate. These improvements are now provided by the present invention.